1. Field of the Invention
This invention relates to an optical transmission module for high-speed communications and, particularly, to an optical transmission module that has a good heat radiation property and can prevent problems caused by thermal expansion.
2. Description of the Related Art
FIG. 5 is a schematic side view showing a conventional optical transmission module.
As shown in FIG. 5, the optical transmission module 101 is composed of: an optical module 102 with an optical element (not shown) built therein; a receptacle portion 103 which is connected to one side of the optical module 102 to pass through a light axis of the optical element; a circuit board 105 to which a lead 104 extended on the opposite side of the optical module 102 is soldered and on which a control circuit (not shown) is mounted; and a housing 106 in which the optical module 102, the receptacle portion 103 and the circuit board 105 are placed. The optical transmission module 101 called “optical transceiver” is known that may be composed such that an optical module with an optical element for light emission built therein and an optical module with an optical element for light reception built therein are in parallel placed in one housing 106. Also, the optical module 102 is known that may be composed such that a peltiert element is placed in a metallic package with an optical element built therein to cool down the optical element.
The assembly process of the optical transmission module 101 can be simplified by previously integrating the optical module 102 and the receptacle portion 103. Also, the assembly process of the optical transmission module 101 can be simplified by placing the optical module 102 and the circuit board 105 in the housing 106 after soldering the lead 104 of the optical module 102 to the circuit board 105.
The housing 106 of the optical transmission module 101 is provided with a window 107 into which a connector (not shown) of an optical fiber as a transmission line is inserted. The connector is connected through the window 107 to the receptacle portion 103 to allow the optical coupling between the optical fiber and the optical module 102.
Conventionally, when the optical module 102, the receptacle portion 103 and the circuit board 105 are placed in the housing 106, the optical module 102 is not fixed to and not in contact with the housing 106 while the receptacle portion 103 and the circuit board 105 are fixed to the housing 106.
For example, as shown in FIG. 6, the housing 106 is composed of divided members, i.e., a housing bottom member 108 and a housing top member 109, which are provided with protrusions 111 and 112, respectively, to compose a receptacle portion supporting member 110, and the protrusions 111 and 112 are provided with a groove into which a flange 113 of the receptacle portion 103 is fitted. The receptacle portion 103 is formed cylindrical and the flange 113 protrudes radially from the outer circumference of the receptacle portion 103. Thus, by forming the groove of the protrusions 111 and 112 to be fitted to the flange 113, the receptacle portion 103 can be positioned and fixed in the optical axis direction and in two directions, i.e., vertical and lateral directions of the housing 106, perpendicular to the optical axis direction.
The reason why the optical module 102 is not fixed to the housing 106 is that it is intended to prevent the optical module 102 and the receptacle portion 103 from being subjected to a stress caused by the difference between the linear expansion coefficient of the optical module 102 and the receptacle portion 103 and the linear expansion coefficient of the housing 106 in the optical axis direction.
The reason why the optical module 102 is not in contact with the housing 106 is that the dimensions of the protrusions 111, 112 are set to minimize the error ΔH between a height H0 from the housing 106 bottom to the connector optical axis C0 and a height H1 from there to the receptacle portion 103 optical axis C1 and, further, the receptacle portion 103 is securely supported thereby and, therefore, a strain may be applied to the optical module 102 if the optical module 102 is in contact with the housing 106.
On the other hand, the housing bottom member 108 is provided with a boss 114 standing toward the circuit board 105. The boss 114 is provided with a female screw hole 115 formed on its upper portion, and the circuit board 105 is provided with a through-hole 116 corresponding to the female screw hole 115. Thus, by screwing a screw 117 through the through-hole 116 into the female screw hole 115, the circuit board 105 can be fixed to the housing 106. The screw-fixed position is set to be near an edge 118 of the circuit board 105(i.e., far from the optical module 102) on the ground that the edge 118 is to be inserted into a circuit board connector (not shown).
For example, Japanese patent publication No. 2004-103743 discloses an equivalent to the optical transmission module 101.
In high-speed communications, it is desired that heat is efficiently radiated from the optical module 102 to the housing 106 since the amount of heat generated from the optical element is large.
However, since the optical module 102 is not fixed to and not in contact with the housing 106 while the receptacle portion 103 is fixed to the housing 106, heat is not radiated directly from the optical module 102 to the housing 106. Namely, heat of the optical module 102 is flown through the receptacle portion 103 and the protrusions 111, 112 to the housing 106. Thus, the efficiency of heat radiation is not good.
In order to improve the heat radiation, a flexible heat radiation sheet 119 may be disposed between the optical module 102 and the housing 106. Thereby, heat can be radiated through the heat radiation sheet 119, and the optical module 102 can be loosely restricted without applying a strain to the optical module 102 since the heat radiation sheet 119 is flexible. However, as the communication speed increases, the optical module 102 must have a very large heat generation and therefore it becomes difficult to address such a heat generation by the heat radiation means through the receptacle portion 103 and the heat radiation sheet 119.
On the other hand, in the conventional structure that the receptacle portion 103 is fixed to the housing 106 and the circuit board 105 is fixed to the housing 106 near the edge 118, a stress is likely to concentrate at the lead 104 or a portion where the lead 104 is soldered when the circuit board 105 thermally expands in the optical axis direction. As a result, the lead 104 may be disconnected.